How do you convert q to ΔH in kJ mol⁻¹?

Convert J to kJ, divide by moles of the limiting reagent and add the sign.

What sign has ΔH if the temperature rises?

Negative: the reaction is exothermic.

Why is a polystyrene cup with a lid used?

It is a poor conductor of heat, so less heat is lost to the surroundings.

What mass is used in q = mcΔT when 50.0 cm³ of acid is mixed with 50.0 cm³ of alkali?

100.0 g, the mass of all the solution.

Why is a cooling-curve correction needed?

Heat is lost while the reaction happens, so the highest temperature recorded is too low.

How is a cooling-curve correction done?

Extrapolate the cooling line back to the time of mixing and use that temperature.

How is the mass of fuel burned found in a spirit burner experiment?

Weigh the burner before and after the experiment and subtract.

Why are experimental ΔcH values less exothermic than data-book values?

Heat loss to the surroundings, incomplete combustion and evaporation of the fuel.

How do you find a percentage uncertainty?

Absolute uncertainty ÷ measured value × 100.

Why is the uncertainty in ΔT twice the thermometer reading uncertainty?

ΔT is found from two readings, so their uncertainties add.

Name two ways to reduce heat loss in a combustion experiment.

Use a draught shield (and lid) and place the flame close to the can.

CalorimetryEdexcel International A Level Chemistry: Flashcards

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What is the equation for energy transferred in calorimetry?

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What is the equation for energy transferred in calorimetry?: q = m × c × ΔT, with q in J, m in g and ΔT in °C.
What is the specific heat capacity of water?: 4.18 J g⁻¹ °C⁻¹.
How do you convert q to ΔH in kJ mol⁻¹?: Convert J to kJ, divide by moles of the limiting reagent and add the sign.
What sign has ΔH if the temperature rises?: Negative: the reaction is exothermic.
Why is a polystyrene cup with a lid used?: It is a poor conductor of heat, so less heat is lost to the surroundings.
What mass is used in q = mcΔT when 50.0 cm³ of acid is mixed with 50.0 cm³ of alkali?: 100.0 g, the mass of all the solution.
Why is a cooling-curve correction needed?: Heat is lost while the reaction happens, so the highest temperature recorded is too low.
How is a cooling-curve correction done?: Extrapolate the cooling line back to the time of mixing and use that temperature.
How is the mass of fuel burned found in a spirit burner experiment?: Weigh the burner before and after the experiment and subtract.
Why are experimental ΔcH values less exothermic than data-book values?: Heat loss to the surroundings, incomplete combustion and evaporation of the fuel.
How do you find a percentage uncertainty?: Absolute uncertainty ÷ measured value × 100.
Why is the uncertainty in ΔT twice the thermometer reading uncertainty?: ΔT is found from two readings, so their uncertainties add.
Name two ways to reduce heat loss in a combustion experiment.: Use a draught shield (and lid) and place the flame close to the can.

Exam questions on Calorimetry

A student measures the enthalpy change of neutralisation. She mixes 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid with 50.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution in a polystyrene cup with a lid, stirring gently. Both solutions start at the same temperature and the highest temperature reached is 6.8 °C higher. Assume that the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ °C⁻¹.
Calculate the energy transferred to the solution, in J, and state one assumption made in your calculation.2 marks
A student determines the standard enthalpy change of combustion of ethanol, C₂H₅OH (Mr = 46.0). She heats 200.0 g of water in a copper can using a spirit burner and measures a rise in temperature of 25.0 °C. The mass of the burner falls by 0.92 g. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹. The data-book value for ethanol is –1367 kJ mol⁻¹.
The experimental value is less exothermic than the data-book value. Suggest two reasons for this.2 marks
A student measures the enthalpy change for the reaction Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). She puts 50.0 cm³ of 0.200 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup and records its temperature every 30 seconds for 2 minutes. At 2.5 minutes she adds an excess of zinc powder (without taking a reading) and carries on recording the temperature every 30 seconds until 8 minutes, stirring throughout. She plots temperature against time.
Explain why a cooling-curve correction is needed and describe how the student uses her graph to find the correct temperature change.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).